I obviously know very little about your relationship, so take this with a grain of salt. But it seems to me that you are pretty generous with her, at least when it comes to loaning her your car. That sort of thing can be used as a bargaining chip to encourage better behavior. The next time she calls and asks to borrow your car, maybe you should say, "You know what? No. I don't appreciate you telling my kids that I am mean to their mom and that my girlfriend is a bad person. You want favors from me, act like a decent human being."
Perhaps you're reluctant to do that because depriving her of your car would, for example, deprive your kids of a trip to the mountains. And yeah, that sucks. But maybe watching their mom walk all over their dad is worse for them than missing a day at the mountains.
pinballwizard might be pushing it a bit too far, but it sounds like there may be some truth to that, "because you let her," business.